1. Field of the Invention
The present invention relates to an ink jet recording apparatus.
2. Related Background Art
The recording apparatus for recording on a recording medium conventionally employs various recording methods such as the wire dot method, thermal recording method, thermal transfer recording method, ink jet method, etc. Among these methods, the ink jet recording method for forming a record on a recording medium by discharging small ink droplets from a discharge port (nozzle) is widely employed in recent years because it is a non-impact recording method with various advantages such as scarce noise generation at recording, and capability of executing high-density and high-speed recording on various recording media.
An ink jet recording apparatus is generally provided with an ink jet recording head, means for conveying the recording medium, and control means for controlling these components. The method for generating energy for ink discharge from the nozzle of the ink jet recording head can be, for example, pressurization of ink with an electromechanical converting element such as a piezo element, bubble generation by irradiation with electromagnetic waves for example from a laser, bubble generation by liquid heating with an electrothermal converting element such as a heat generating resistor, etc. Among these, a method for discharging ink droplets by thermal energy (bubble jet method) can achieve recording of high resolution because the energy generating means can be arranged at a high density. Particularly, an ink jet recording head utilizing an electrothermal converting element as the energy generating means can be made compact and provides advantages of easily achieving high-density configuration and low manufacturing cost, utilizing the IC technology and the microfabrication technology showing remarkable progress and improvement in reliability in the semiconductor area.
The ink jet recording apparatus representing the background technology and shown in FIG. 6 is a recording apparatus of serial type, in which the recording head is mounted on a carriage (not shown) and the recording operation is executed by the movement of such carriage, employing so-called tube supply system in which the recording head is connected with a main tank through a tube. Such ink jet recording apparatus is provided with a main tank (ink tank) 104 for containing ink, a recording head 101 for discharging ink droplets by thermal energy, an ink supply unit 105 and an ink supply tube 106 for ink supply from the main tank 104 to the recording head 101, an air discharge tube 110a, a shut-off valve 110b and an air discharge pump 110c for opening the recording head 101 to the air, and a recovery unit 107 for a recovery process for the recording head 101.
At first there will be explained the schematic configuration of the recording head 101. A discharge nozzle 101g in the recording head 101 is composed of a fine hole. The nozzle 101g is not provided with a valve mechanism, and ink leakage from the nozzle 101g or air intrusion therein is prevented by maintaining the interior of the nozzle at a negative pressure, thereby forming an ink meniscus at the front end of the nozzle. More specifically, since the nozzle 101g is open to air and the aperture of the nozzle 101g is positioned downwards, the interior thereof has to be maintained at a negative pressure in order to prevent ink leakage therefrom. On the other hand, an excessively large negative pressure causes air to enter the nozzle 101g, thereby disabling the ink discharging operation. Therefore, in order to maintain the interior of the recording head 101 at an appropriate negative pressure state, the recording head 101 is so positioned that the aperture of the nozzle 101g is higher by a height H than the ink liquid level in an ink chamber 105f (to be explained later), thereby maintaining the interior of the recording head 101 at a negative pressure corresponding to a water head of a height H. Thus, the nozzle 101g is maintained in a state filled with ink by forming a meniscus at the aperture.
The ink discharge is executed by pushing out the ink in the nozzle 101g by film boiling energy generated by an unrepresented heater (heat generating resistor) positioned in the vicinity of the nozzle 101g. After the ink discharge, the ink is replenished into the nozzle 101g by the capillary force thereof and is thus sucked up from time to time from the main tank 104 through the ink supply tube 106. Such ink discharge and ink supply (refilling) are repeated.
In the interior of the recording head 101, there are provided a filter 101c of fine mesh structure for preventing clogging of the fine hole of the nozzle 101g with particles, a flow path 101f finely branched for connecting the filter 101c with the nozzles 101g, and a sub tank 101b for containing a predetermined amount of ink at the upstream side of the filter 101c, whereby the ink flowing in from the ink supply tube 106 is supplied to the nozzle 101g. 
In the following there will be explained the schematic configuration of the main ink 104 and the ink supply unit 105. The configuration is substantially same as that disclosed, for example, in the Japanese Registered Patent No. 2929804, wherein a hollow ink supply needle 105a and a hollow air introducing needle 105b fixed to the ink supply unit 105 penetrate a connector 104b at the bottom of the main tank 104 and enter the main tank 104. Inside the ink supply unit 105, there is provided an ink tank chamber 105f which is open to the air by an air communicating aperture 105g, and the needles 105a, 105b are positioned therein so as to be immersed, with different lower end heights in the ink. The bottom of the ink chamber 105f communicates with the ink supply tube 106, and, along with the ink consumption, the ink in the ink chamber 105f decreases whereby the lower end of the air introducing needle 105b is separated from the ink and is exposed to the air. Thus, the air introduced from the lower end of the air introducing needle 105b into the main tank 104 and the ink in the main tank 104 flows to the ink chamber 105f. When the liquid level in the ink chamber 105f rises by such ink flow, the lower end of the air introducing needle 105f is again immersed in the ink, thereby terminating the air introduction into the main tank 104 and the ink flow into the ink chamber 105f. In this manner the ink in the ink tank 104 is gradually taken out.
In the lower part of the main tank 104, an electrode 104e is provided in contact with the ink, thus in electric conduction with a contact 104j provided in the ink supply unit 105. Between the contact 105j and the air introducing needle 105b, there is connected a detection circuit including a detector 105h for measuring the electric resistance of the ink. The presence or absence of the ink can be detected by measuring the electric resistance of the ink by such detection circuit.
In the following there will be explained the air discharge tube 110a, the shut-off valve 110b and the air discharge pump 110c. In the sub tank 101b of the recording head 101, there may be accumulated air that permeates through a resinous material, for example, of the ink supply tube 106 or is dissolved in the ink. Therefore, thus accumulated excessive air is periodically discharged, together with the ink, from a lateral portion of the sub tank 101b, by suction with the air discharge tube 110a and the air discharge pump 110c. Upon completion of the air discharge, the air discharge path is closed by the shut-off valve 110b. 
In the following there will be explained the recovery unit 107. In case the discharge nozzle 101g is clogged with viscosified ink or with excessive bubbles generated at the ink discharge, the recovery unit 107 is used for eliminating such viscosified ink or bubbles by contacting a suction cap 107a with the recording head 101, and sucking the ink, together with the viscosified ink and bubbles, strongly from the nozzle 101g by a suction pump thereby recovering the function of the recording head 101.
In the ink jet recording apparatus of the aforementioned background technology, there is known a phenomenon, in case of ink discharge for recording by bubble generation of the air dissolved in the ink by heat generation in a heater corresponding to the nozzle 101g, that bubbles are gradually accumulated in the flow path 101f by fission of the generated bubbles and accumulation thereof in the flow path 101f under the filter 101c or by gathering of fine bubbles present in the ink by a temperature increase around the heater.
In the configuration of the aforementioned background technology, since the flow path 101f is narrowly formed, the ink flow tends to become stagnant therein so that the movement of the bubble is retarded. The strong suction by the recovery unit 107 increases the ink flow speed whereby the ink and bubbles in the flow path 101f can be discharged, but, if the bubble grows to a size completely interrupting the flow path 101f, the ink supply to the nozzle 101g is hindered, so that the suction by the recovery unit 107 has to be executed frequently to discharge the bubble before it grows excessively. Therefore, the amount of ink wasted at each suction inevitably increases.
On the other hand, if the flow path 101f is formed thicker so as not to be interrupted or clogged by the bubble, the bubble rises to the upper part of the flow path 101f and the suction, even if executed strongly, from the nozzle 101g by the recovery unit 107 can only suck out the ink and cannot discharge the bubble by suction. Also, since the filter 101c has a fine mesh structure, the ink entering each pore of the mesh forms a meniscus therein which cannot be penetrated by the air. Thus the bubbles cannot escape to the sub tank 101b but accumulate in the upper part of the flow path 101f. Such bubble accumulation increases the volume occupied by the air in the flow path 101f, thus leading to an ink amount decrease therein, which leads to the exposure, to the air, of the ink supply aperture at the upper face of the nozzle 101g, eventually resulting in a situation in which the ink supply thereto is disabled.
Particularly in recent years, as a result of increase in the number of nozzles and in the driving rate (ink discharge at a higher drive frequency) in the recording head, the bubble generation at the printing operation is increasing in general. The ink consumption per unit time becomes therefore larger, so that the fine flow path as employed in the background technology leads to a larger pressure loss, thus resulting in a discharge failure.